choleric
Easily angered and quick to lose one’s temper.
Choleric describes someone who gets angry quickly and easily, like a pot of water that boils over at the slightest increase in heat. A choleric person has a short fuse: minor frustrations that others might shrug off can send them into a rage. If your friend becomes choleric when losing at checkers, slamming pieces down and shouting, you've seen this temperament in action.
The word comes from ancient medicine, when doctors believed four fluids called humors controlled personality. They thought too much yellow bile (called choler) made people hot-tempered and irritable.
You might encounter choleric in older literature describing characters who fly off the handle. A choleric king might harshly punish advisors for bringing bad news. A choleric coach might scream at players for tiny mistakes. The word suggests a personality defined by quick, intense reactions to frustration.
Note that choleric specifically means prone to anger, different from being energetic or passionate. A passionate scientist excitedly explaining her research isn't choleric, but one who yells at colleagues for questioning her methods might be.